Portable USB charger



FIG. 1 is a front, top, right side perspective view of a portable USB charger embodying my new design;

FIG. 2 is a rear, bottom, left side perspective view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a left side elevational view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines shown in the drawings are included for the purpose of illustrating environmental structure and form no part of the claimed design. 

The ornamental design for a portable USB charger, as shown and described. 